1. Field of the Invention
The present invention relates to a developing device used in an electrophotographic field, wherein an electrostatic latent image is visually developed by using a non-magnetic type one-component developer.
2. Description of the Related Art
As is well known, an electrophotographic printer carries out the processes of: producing a uniform distribution of electrical charges on a surface of an electrostatic latent image carrying body; forming an electrostatic latent image on the electrically charged surface of the electrostatic latent image carrying body by optically writing an image thereon by using a laser beam scanner, an LED (light emitting diode) array, an LCS (liquid crystal shutter) array or the like; visually developing the electrostatic latent image with a developer, i.e., toner, which is electrically charged to be electrostatically adhered to the electostatic latent image zone; electrostatically transferring the developed visible image to a sheet or paper; and fixing the transferred image on the sheet or paper. Typically, the electrostatic latent image carrying body may be an electrophotographic photoreceptor, usually formed as a photosensitive drum, having a cylindrical conductive substrate and a photoconductive insulating film bonded to a cylindrical surface thereof.
In the developing process, a two-component developer composed of a toner component (colored fine synthetic resin particles) and a magnetic component (magnetic fine carriers) is widely used, as this enables a stable development of the latent image. Note, typically the toner particles have an average diameter of about 10 .mu.m, and the magnetic carriers have a diameter ten times larger than the average diameter of the toner particles. Usually, a developing device using the two-component developer includes a vessel for holding the two-component developer, wherein the developer is agitated by an agitator provided therein. This agitation causes the toner particles and the magnetic carriers to be subjected to triboelectrification, whereby the toner particles are electrostatically adhered to each of the magnetic carriers. The developing device also includes a magnetic roller, provided within the vessel as a developing roller in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the photosensitive drum. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon. In this developing device, a ratio between the toner and magnetic components of the developer body held in the vessel must fall within a predetermined range, to continuously maintain a stable development process. Accordingly, the developing device is provided with a toner supplier from which a toner component is supplied to the two-component developer held in the vessel, to supplement the toner component as it is consumed during the development process, whereby the component ratio of the two-component developer held by the vessel is kept within the predetermined range. This use of a two-component developer is advantageous in that a stable development process is obtained thereby, but the developing device per se has the disadvantages of a cumbersome control of a suitable component ratio of the two-component developer, and an inability to reduce the size of the developing device due to the need to incorporate the toner supplier therein.
A one-component developer is also known in this field, and a developing device using same does not suffer from the above-mentioned disadvantages of the developing device using the two-component developer, because the one-component developer is composed of only a toner component (colored fine synthetic resin particles). Two types of the one-component developer are known; a magnetic type and a non-magnetic type. A developing device using the magnetic type one-component developer can be constructed in substantially the same manner as that using the two-component developer. Namely, the magnetic type one-component developer also can be brought to the surface of the photosensitive drum by a rotating magnetic roller as in the developing device using the two-component developer. The magnetic type one-component developer is suitable for achromatic color (black) printing, but is not suitable for chromatic color printing. This is because each of the toner particles of which the magnetic type one-component developer is composed includes fine magnetic powders having a dark color. In particular, the chromatic color printing obtained from the magnetic type one-component developer appears dark and dull, due to the fine magnetic powders included therein. Conversely, the non-magnetic type one-component developer is particularly suitable for chromatic color printing because it does not include a substance having a dark color, but the non-magnetic type one-component developer cannot be brought to the surface of the photosensitive drum by the magnetic roller as mentioned above.
A developing device using the non-magnetic type one-component developer is also known, as disclosed in U.S. Pat. Nos. 3,152,012 and 3,754,963. This developing device includes a vessel for holding the non-magnetic type one-component developer, and a conductive solid rubber roller rotatably provided within the vessel as a developing roller in such a manner that a portion of the solid rubber developing roller is exposed therefrom and faces the surface of the photosensitive drum. The solid rubber developing roller may be formed of a conductive silicone rubber material or a conductive polyurethane rubber material, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 60-12627 and Japanese Unexamined Patent Publications (Kokai) No. 62-118372 and No. 63-189876. When the conductive solid rubber developing roller is rotated within the body of the non-magnetic type one-component developer held by the vessel, the toner particles composing the non-magnetic type one-component developer are frictionally entrained by the surface of the solid rubber developing roller to form a developer layer therearound, whereby the toner particles can be brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon. The developing device further includes a blade member engaged with the surface of the developing roller, to uniformly regulate a thickness of the developer layer formed therearound so that an even development of the latent image can be carried out. The blade member also serves to electrically charge the toner particles by a triboelectrification therebetween. In this developing device, the development process is carried out in such a manner that, at the area of contact between the photosensitive drum and the conductive solid rubber developing roller carrying the developer layer, the charged toner particles are electrostatically attracted and adhered to the latent image due to a developing bias voltage applied to the conductive solid rubber developing roller.
Japanese Unexamined Patent Publication (Kokai) No. 62-96981 discloses a developing device using the one-component developer, in which a rubber blade member is used to regulate a thickness of the developer layer formed around the developing roller. This rubber blade member is in the form of a rectangular plate element and has a width substantially equal to a length of the developing roller. The rubber blade member is slidably received in a guide holder member, and is resiliently pressed against the developing roller. A bottom end face of the blade member, which is in contact with the surface of the developing roller, is formed as a slant face so that the blade member has acute and obtuse angle edges at the bottom end face thereof, and the blade member is engaged with the rotating developing roller in such a manner that the acute angle edge thereof penetrates the developer layer formed around the developing roller. With this arrangement, even though the developing roller is eccentrically rotated (note, a slight eccentric rotation of the developing roller is permissible as a tolerance), the contact between the slant end face of the blade member and the surface of the developing roller is maintained because the blade member is resiliently pressed against the developing roller, and thus a regulation of the developing layer thickness can be ensured by the penetration of the acute angle edge of the blade member to the developer layer.
Nevertheless, the above-mentioned rubber blade member has a disadvantage of a susceptibility to mechanical damage, i.e., the acute angle edge of the blade member can be easily chipped away, and obviously, an even regulation of the developer layer thickness cannot be ensured by a chipped acute angle edge of the blade member. Also, in the developing device disclosed in the above-mentioned Publication (Kokai) No. 62-96981, the excess toner particles removed from the developer layer by the blade member are not prevented from entering the guide holder member in which the blade member is slidably received, so that the blade member may become immovable in the guide holder member, and of course, when the blade member is immovable in the guide holder member, it is impossible to properly regulate the developer layer thickness. Furthermore, when a frictional force between the blade member and the developing roller with the developer layer becomes large, due to variations in the temperature and air moisture content, the blade member may be vibrated for the reasons stated hereinafter in detail, and thus variations of the regulated developer layer thickness appear.
The blade member also serves to electrically charge the toner particles by a triboelectrification therebetween, as mentioned above. In this case, the blade member must be constituted in such a manner that the toner particles forming the regulated developer layer can be given a charge distribution that will produce a proper development of an electrostatic latent image, since if this is not ensured, an electrophotographic fog may appear during the development process and the developer be wastefully consumed for the reasons stated hereinafter in detail.